For years, researchers have known that marathons cause temporary spikes in biomarkers of organ stress—things like creatinine (a kidney function marker) and gut cell damage. But most studies haven’t measured these changes before and after a marathon under real-world race conditions. And the role of hydration—or sex differences—has remained murky.
A new study from researchers at the University of Arkansas and collaborators set out to investigate this further. The researchers collected blood and urine from 72 Boston Marathon finishers in 2024—half male, half female, with an average age of 50 and a finish time around 3:45. Participants provided baseline blood samples at the race expo and urine samples before and after the marathon or an analysis of markers of gut injury, kidney stress, muscle damage (i.e., creatine kinase), and hydration. They also compared responses between males vs. females and runners who stayed hydrated or finished the race dehydrated.
The 2024 race was mild by Boston standards: 19°C (66°F) and 47% humidity, warm enough to pose a heat load but not extreme. Still, the results showed that marathon running—even in moderate conditions—places an enormous stress on key organ systems.
Every single organ stress marker spiked significantly after the marathon. Kidney stress markers rose sharply—88% of runners exceeded the clinical threshold for acute kidney injury risk, and 96% showed a creatinine rise of at least 0.30 mg/dL, levels that would raise red flags in a medical setting.
Gut injury markers also increased, with three-quarters of runners exceeding the clinical threshold for intestinal cell injury. Creatine kinase (the marker of muscle damage) also rose, confirming substantial muscular stress (probably from Boston’s downhill segments). Despite measurable shifts in hydration markers, only about a third of runners were technically dehydrated, and hydration status didn’t alter the organ stress responses. Neither sex nor age influenced the results, and finish time didn’t predict biomarker changes either. Interestingly, gastrointestinal symptoms like nausea or cramps didn’t correlate with these biomarker spikes, showing that the gut can sustain injury even without obvious discomfort.
As a marathon runner, do these results concern me? Hardly. Many, if not all, of these changes likely resolve within 24-48 hours of a race, especially in the context of adequate nutrition, hydration, and recovery. I find it interesting that hydration didn’t prevent these effects, suggesting that reduced blood flow to the gut and heat strain are major contributors to organ stress.
In any case, this study highlights the need for true recovery after a marathon, especially avoiding back-to-back hard efforts. Look at post-race recovery as more than muscle repair, but as an opportunity to let your internal systems (and your mind!) rebound.
Thanks for sharing this, it’s a great insight and can guide better post race for recovery strategy. It certainly has me thinking about my athletes who are doing 2 week bikepacking races with heavy hike-a-bike sections. It would be interesting to know whether more eccentric weight training for the quads had an impact on the markers by preventing muscle breakdown on the downhill sections.
Aren’t there also studies showing people with elevated troponin at the marathon finish line? Context is everything.